In the unpublished German patent application No. 10 2010 056 167.3 a belt drive of this species is disclosed, wherein in a pulley plane a plurality of units, but at least one belt starter-generator and possibly an air conditioner compressor and others, are operatively connected by means of a belt to the crankshaft of an internal combustion engine of a motor vehicle. To adjust the torques transmissible from and to the latter, and the speeds of rotation which occur thereby, a shiftable planetary gear unit is integrated into the belt drive, whose sun wheel is connected to the belt and therefore ultimately to the belt starter-generator, while the ring gear can be connected fixedly with respect to the housing by means of a brake, and the web which accommodates the planet carrier is operatively connected to the crankshaft in a rotationally fixed connection. Situated in the power path between sun wheel and web is a switchable freewheeling mechanism, which is switchable to blocking and over-rolling in both directions of transmission, depending on the switched state. Depending on the switched state of the brake and of the freewheeling mechanism, the switched states of the planetary gear set result, in order to activate corresponding transmission ratios and flows of torque for example in generator mode, in which the belt starter-generator is driven by the crankshaft, in starting operation of the internal combustion engine, in which the crankshaft is driven by the belt starter-generator, or in stationary air conditioning operation, in which the air conditioner compressor is driven by the belt starter-generator while the crankshaft is standing still.
In this case, the brake and the freewheeling mechanism are actuated by a single actuator, which is, for example, an electric motor with a spindle gear unit. The actuator moves the actuating devices of the brake and of the freewheeling mechanism jointly along a travel distance. In particular for reasons of cost, the travel distance is monitored by a relative distance sensor, which must be calibrated and referenced regularly.